Polymorphism, Sulfapyridine

Sample

Sulfapyridine

Application

Active ingredient

Conditions

Measuring cells: DSC820, FP82 microscope hot stage

Pan: DSC: Aluminum 40 µl, hermetically sealed, FP82: Sample holder with cover glass

Sample preparation: As received, no preparation

DSC measurement: Shock-cooled from the melt. Then heating from 40 °C to 200 °C at 5 K/min

Atmosphere: Nitrogen, 50 cm³/min

Interpretation

Shock-cooling (rapidly cooling) the sample to a temperature below the glass transition temperature T_g before performing a heating run is often the best method of detecting polymorphism. According to Oswald, the metastable phase (B, if one exists at all) crystallizes above the glass transition T_g (A). This is transformed on further heating first monotropically (C, solid-solid transition) and then, after the melting of the metastable phase (D), by crystallization from the liquid phase (E) to the stable modification. This then finally melts (F).

It is known from other DSC measurements, that sulfapyridine exhibits further modifications.

Evaluation

Photographs were taken of the crystal modifications at different temperatures using the FP82 hot stage microscope in order to gain additional information about the polymorphic behavior of the substance. The following photographs correspond to the crystal modifications at about 110 °C and 180 °C.

Conclusion

The possibility of a substance exhibiting polymorphism is best investigated by first of all melting it most carefully (rapidly, under nitrogen) and then immediately shock-cooling it under its glass transition temperature (rule of thumb: T_g is roughly 3/4 T_f, both temperatures are in Kelvin). Afterwards, the different modifications can be observed by slowly heating the sample in the DSC cell.

_{Polymorphism, Sulfapyridine | Thermal Analysis Application No. HB831 | Application published in METTLER TOLEDO TA Application Handbook Pharmaceuticals}